1. Field of the Invention
This invention relates to allocation of data storage and more particularly relates to allocation of data storage at a storage device.
2. Description of the Related Art
Computers and storage systems (such as storage area networks (SANs)) often make use of cache to speed data writing and retrieval. Due to the cost associated with devices that have sufficient speed to act as effective cache, the amount of memory available as cache is often limited. Solid state storage (such as Flash) may be used to provide relatively affordable, high-speed and fast caching memory. Hard drives may be used as cache in front of slower media such as tape.
However, many types of storage media degrade with time. For example, Flash memory has a limited life cycle; the reliability of the Flash memory degrades as the read-write cycles increase. The device becomes increasingly unreliable as a storage device with use, and eventually dies completely. The limited life cycle of physical devices (whether solid state memory or hard disk) reduces its ability to act as effective cache. Since an effective storage system should be able to guarantee, with a high degree of reliability, that data written to the system will not be lost, using a cache device that degrades with time can cause difficulty. The possibility of losing data is an impediment to the acceptance and implementation of nonvolatile memory as cache. It can also increase costs as administrators need to replace cache devices (even if they may be functional) because the risk of data loss associated with continued use of the cache device is simply too high. Furthermore, attempting to write data to a cache device, and read data from a cache device, when that data is no longer trustworthy, unnecessarily increases traffic and overhead on a storage system.